Transmitter circuit with integrated power control

ABSTRACT

An integrated circuit device, set forth by way of example and not limitation, includes an IC package provided with a plurality of leads and enclosing: a) a buffer amplifier, b) a switching-mode power amplifier having an input coupled to the output of the buffer amplifier and having an output coupled to at least one of the plurality of leads, and c) a digital controller. A method, set forth by way of example and not limitation, for controlling the power output of a RF transmitter circuit without the need for an attenuator includes developing a signal source, applying the signal source to a buffer amplifier to provide an amplified signal, applying the amplified signal to a switching-mode power amplifier to provide a power output signal, and controlling a gain of the switching-mode power amplifier in response to a digital command.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. Ser. No. 13/710,420, filed onDec. 10, 2012 and issued as U.S. Pat. No. 8,675,770 on Mar. 18, 2014,which is a continuation of U.S. Ser. No. 12/642,690, filed on Dec. 18,2009, and issued as U.S. Pat. No. 8,331,486 on Dec. 11, 2012, both ofwhich are incorporated herein by reference.

BACKGROUND

There is a need for low-powered radio frequency (“RF”) transmitters forsuch purposes as remote control or remote monitoring. For example,low-powered RF transmitters are used in garage door openers, automaticmeter reading (AMR), remote keyless entry (RKE) and home automation. Inorder to accommodate this need, the Federal Communications Commission(“FCC”) has set aside certain radio frequency ranges (“bands”) known as“unlicensed ISM radio bands.” The maximum permitted power oftransmitters operating in various ISM radio bands can vary. Exceedingthe designated maximum power for a particular band is a violation of FCCrules in the U.S. and ETSI rules in Europe.

The need to set a maximum power for RF transmission has been addressedin the prior art. For example, attenuators have been used to reduce RFpower. However, attenuators are expensive and wasteful of energy. Thisis particularly problematic with battery operated devices, such asremote controls.

In the past, cost constraints have dictated relatively inexpensiveamplifiers to be used in battery-powered RF devices. For example,inexpensive class A amplifying devices have been widely used. Class Aamplifiers operate over the whole of the input cycle such that theoutput signal is an exact scaled-up replica of the input with noclipping. However, class A amplifiers are not very efficient and onlyhave a theoretical maximum of 50% efficiency.

More recently, more efficient switching amplifiers have been used.Switching amplifiers are referred to as class E/F amplifiers and arehighly efficient.

FIG. 1 illustrates a transmitter system 10 of the prior art whichincludes a high efficiency switching amplifier. The transmitter system10 includes an integrated circuit (IC or “chip”) 12, designated by thebroken lines, having the switching amplifier and a number of “off-chip”components such as a microcontroller (μC) 14, memory 16, RF choke(inductor) 18, resistor 20, bypass capacitor 22, attenuator 24, matchingnetwork 26 and a load R_(L) (e.g. an antenna). The microcontroller 14and memory 16 are used to digitally control the frequency of an outputsignal developed at an output 28 of the integrated circuit 12 and theresistor 20 is used to control the power of the output signal at output28. The resistor 20 limits the maximum power that is delivered to theload (to meet FCC requirements) and the matching network 26 matches theimpedance of the output signal to the load R_(L). In transmitter systemswhich do not use a resistor 20 to control the power, and the poweroutput is fixed, an optional attenuator 24 can be coupled between thematching network 26 and load R_(L) to prevent excessive powertransmission. However, the optional attenuator 24 is not required if thepower level is properly controlled by a resistor.

Integrated circuit 12 includes, among other components, a signal source30, a buffer amplifier 32 and a switching-mode power amplifier 34. Thefrequency of the signal source 30 can be digitally controlled bymicrocontroller 14. For example, the signal source 30 can be aprogrammable phase lock loop (PLL). The buffer amplifier 32 provides afirst stage of amplification and signal conditioning, and theswitching-mode power amplifier 34 boosts the power which is developed atoutput 26. Integrated circuits similar to integrated circuit 12 havebeen developed by Maxim Integrated Products of Sunnyvale Calif. as, forexample, product numbers MAX1472, MAX7044, MAX1479, MAX703x, MAX7057 andMAX7058.

The series connection of inductor 18 and resistor 20 between the output28 of integrated circuit 12 and a voltage source V_(DD) allows the powerof the signal at output 28 to be controlled. This is accomplished byvarying the resistance of resistor 20 to change the voltage level at theoutput 28 and thus the drain of power transistor (e.g. a MOSFET powertransistor) 34. Due to the inductor 18, the voltage at the output 28 canvary up to a maximum of twice that of the voltage source (2×V_(DD)). Asthe resistance of resistor 20 goes up, the voltage level (and thereforethe power) at output 28 goes down, and vice versa. The bypass capacitor22 shunts high frequency signals to ground.

While the described integrated circuit 12 has many advantages over priorart transmitters using less-efficient amplifiers, there is still roomfor improvement. For example, since the integrated circuit 12 allows thefrequency at the output 28 to be varied, care must be taken to make surethe power output of the integrated circuit 12 does not exceed thoseproscribed by FCC regulations. This requires either an off-chip resistor(such as resistor 20) or an energy-wasteful attenuator 24. Furthermore,since the resistor 20 which determines the power output of theintegrated circuit 12 is “off-chip” and is selected by a systemintegrator, the correct resistance value and the temperaturecharacteristics of the system 10 must be empirically determined.

FIG. 2 is a graph illustrating a typical supply current and output powervs. external resistor curve of an RF transmitter system of FIG. 1. Asnoted above, a problem with this power control method is that anexternal resistor is required and that the gain cannot be changed ‘onthe fly.’ In addition, the control characteristic is very non-linearwith respect to power in dBm.

Amplifiers with a linear-in-dB control characteristic are desirable fora variety of RF system applications including gain control for receiversand level control for transmitters. Some circuit techniques have alreadybeen developed for linear-in-dB power control amplifiers. For example,there are those which utilize the exponential I-V relationship ofbipolar junction transistors (see, for example, U.S. Pat. Nos. 5,200,655and 5,684,431) and there are others that utilize the scaling propertiesof MOS devices (see, for example, U.S. Pat. Nos. 7,391,260 and7,403,071). All of these techniques, however, are only operative withamplifiers that operate in the class A mode.

These and other limitations of the prior art will become apparent tothose of skill in the art upon a reading of the following descriptionsand a study of the several figures of the drawing.

SUMMARY

In an embodiment, set forth by way of example and not limitation, an RFtransmitter circuit with output power control includes a bufferamplifier stage, a switching-mode power amplifier stage having an inputcoupled to an output of the buffer amplifier and having a power output,a DAC having a digital input and an analog output, and an inductorcoupling the analog output of the DAC to the switching-mode poweramplifier stage. In an example alternate embodiment, the analog outputof the DAC is an exponential function of the digital input. In anotherexample alternate embodiment the DAC includes logic including a numberof inputs, a reference voltage source controllable by the logic toprovide a variable reference voltage, a regulator having a first inputcoupled to the variable reference voltage, an electronic valve having acontrol input coupled to an output of the regulator and a first nodecoupled to a voltage source, and a variable voltage divider circuitincludes a series connection of at least two resistors, at least one ofwhich is controllable by the logic, where a node between the at leasttwo resistors is coupled to a second input of the regulator.

In an embodiment, set forth by way of example and not limitation, anintegrated circuit device includes an IC package provided with aplurality of leads and enclosing: a) a buffer amplifier, b) aswitching-mode power amplifier having an input coupled to the output ofthe buffer amplifier and having an output coupled to at least one of theplurality of leads, and c) a controller. In this example embodiment, adigital input of the controller is coupled to at least two of theplurality of leads and output of the controller is coupled to anotherlead to operationally provide a digitally controlled voltage source. Inan example alternate embodiment the DAC is an exponential DAC and in amore specific example alternate embodiment the power amplifier can becontrolled by the DAC to provide linear-in-dB power control of theintegrated circuit device.

In an embodiment, set forth by way of example and not limitation, amethod for controlling the power output of a RF transmitter circuitincludes developing a signal source, applying the signal source to abuffer amplifier to provide an amplified signal, applying the amplifiedsignal to a power amplifier to provide a power output signal, andcontrolling a gain of the power amplifier in response to a digitalcommand. In an example alternate embodiment, the act of controlling thegain includes converting a digital input into an analog control signaland, in a more specific example alternate embodiment, the relationshipbetween the digital command and the power output signal is linear-in-dB.

An advantage of certain embodiments, set forth by way of example and notlimitation, is that RF power levels can be adjusted to appropriatemaximum levels for various radio bands without the need forenergy-wasteful attenuators.

An advantage of certain embodiments, set forth by way of example and notlimitation, is that an RF power level can be simply controlled through adigital input.

An advantage of certain embodiments, set forth by way of example and notlimitation, is that an RF power level can be accurately adjusted in, forexample, discrete steps.

An advantage of certain embodiments, set forth by way of example and notlimitation, is that a RF power can be developed which is stable over asignificant range of temperatures.

An advantage of certain embodiments, set forth by way of example and notlimitation, is that it allows the maintenance of the highest FCCpermitted transmission (“TX”) power at every garage door opener (“GDO”)frequency

An advantage of certain embodiments, set forth by way of example and notlimitation, is that it implements a mathematical function usingfully-integrated linear-in-dB control, which is a highly desired controlcurve for radio frequency power control.

An advantage of certain embodiments, set forth by way of example and notlimitation, is that it utilizes the characteristics of switching-modeamplifiers to retain efficiency as power is backed off.

These and other embodiments and advantages and other features disclosedherein will become apparent to those of skill in the art upon a readingof the following descriptions and a study of the several figures of thedrawing.

BRIEF DESCRIPTION OF THE DRAWINGS

Several example embodiments will now be described with reference to thedrawings, wherein like components are provided with like referencenumerals. The example embodiments are intended to illustrate, but not tolimit, the invention. The drawings include the following figures:

FIG. 1 is a block diagram of a RF transmitter circuit used to illustratevarious embodiments of the prior art;

FIG. 2 is a graph illustrating a typical supply current and output powervs. external resistor curve of an RF transmitter circuit of FIG. 1;

FIG. 3 is a block diagram of an embodiment, set forth by way of exampleand not limitation, of a RF transmitter system with integrated powercontrol;

FIG. 4 is a block diagram of an embodiment, set forth by way of exampleand not limitation, of a DAC of FIG. 3;

FIG. 5 is a graph illustrating a power vs. PA power code curves at twodifferent temperatures for the embodiments of FIGS. 3 and 4; and

FIG. 6 is a graph illustrating efficiency vs. PA power codes for boththe embodiments of FIGS. 3 and 4 and the prior art illustrated in FIG.1.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIGS. 1 and 2 were discussed with reference to the prior art. FIG. 3illustrates, by way of example but not limitation, an RF transmittersystem 36 with integrated power control. It will be noted that thetransmitter system 36 of FIG. 3 is an improvement upon the transmittersystem 10 described with reference to FIG. 1. To the extent thatelements of the transmitter circuits are similar, similar referenceswill be used.

By way of non-limiting example, FIG. 3 illustrates a transmitter system36 which, like transmitter system 10, includes a high efficiencyswitching amplifier. The transmitter system 36 includes an integratedcircuit (IC) 38, designated by the broken lines, having the switchingamplifier. Transmitter system 36 also includes a number of “off-chip”components such as a microcontroller (μC) 14, memory 16, RF choke(inductor) 18, bypass capacitor 22, matching network 26 and a load R_(L)(e.g. an antenna). It should be noted that the transmitter system 36, ofthis example, does not require the external resistor to adjust its powerlevel or an attenuator to limit the power delivered to the load R_(L).Of course, certain off-chip components can be integrated on-chip, andvice versa. The microcontroller 14 and memory 16 can be used todigitally control both the frequency of an output signal developed at apower output 28 of the integrated circuit 38. The matching network 26matches the impedance of the load R_(L) to the rest of the system.

In this example embodiment, integrated circuit 38 includes, among othercomponents, a programmable signal source 30, a buffer amplifier 32 and aswitching-mode power amplifier 34. The frequency of the signal source 30can be digitally controlled by microcontroller 14. For example, thesignal source 30 can be a programmable phase lock loop (PLL). The bufferamplifier 32 provides a first stage of amplification and signalconditioning, and the switching-mode power amplifier 34 boosts the powerwhich is developed at output 26. The design and implementation of signalsource 30, buffer amplifier 32 and switching-mode power amplifier 34 onan integrated circuit is well known to those of skill in the art.

Integrated circuit 38, in this example embodiment, further includes adigital-to-analog converter (DAC) 40. As seen in FIG. 3, the DAC 40 has,as an input, an N-bit word developed by the microcontroller 14, i.e. theDAC 40 has N input lines in this example. Under microcontroller 14control, the voltage output of DAC 40 can be adjusted in 2^(N)increments to develop a voltage PA_(VDD) which is applied to a node 42between the inductor 18 and the capacitor 22. By way of non-limitingexample, if then the power can be adjusted in 2⁵ or 32 increments orsteps. As such, DAC 40 provides a digitally controlled voltage source.However, it must be emphasized that the DAC 40 is just an example of acontroller which can provide a digitally controlled voltage source, andother such controllers are well known to those of skill in the art.

The signal source 30 has, in this example embodiment, an M-bit wordinput that is developed by the microcontroller 14. The frequency of theintegrated circuit 38 can therefore be adjusted in 2^(M) increments. Theconstant M can be greater than, equal to, or less than the constant N.

Since the output voltage PA_(VDD) of DAC 40 is under microcontroller 14control, the relation between the steps in the N-bit word and the powerdeveloped at the output 28 of the integrated circuit 38 can be virtuallyany function. That is, the microcontroller 14 along with programinstructions and, perhaps, lookup tables stored in, for example, memory16 can create a virtually unlimited variety of waveforms that relate theoutput power of the integrated circuit 38 to the N-bit word input intothe DAC 40. By way of non-limiting example, the output power of theintegrated circuit 38 can be programmed to be linear-in-dB with respectto the least significant bits (LSB) of the N-bit word (aka “PowerCodes”).

In an embodiment, set forth by way of example and not limitation, DAC 40is an exponential voltage DAC, with an output power that can beexpressed as:PA _(VDD) =ke ^(qn)  (Equation 1)where, in Equation 1, PA_(VDD) is the D.C. voltage at the output of theDAC, k and q are constants, and n is the DAC setting in leastsignificant bits (“LSBs”). That is, the variable n is used to signify avalue (“Power Code”) of the N-bit word in the range of zero to N−1.

FIG. 4 is a block diagram of an embodiment, set forth by way of exampleand not limitation, of a DAC of FIG. 3 which operates as an exponentialvoltage DAC. In this example embodiment, DAC 40 includes a bandgapvoltage source 44, a scale factor (S) device 46, a regulator 48, atransistor (e.g. MOSFET) device 50, resistors (R2) 52 and (R1) 54, andlogic 56. The node between transistor device 50 and resistor 52 iscoupled to node 42 and develops the voltage PA_(VDD).

In the example embodiment of FIG. 4, the bandgap voltage source 44develops a steady, consistent voltage V_(BG). The bandgap voltage source44 can, by way of non-limiting example, provide a steady voltage of 1.14volts. The scale factor device 46, which is essentially a DC-to-DCup-converter, multiplies the voltage V_(BG) by the scaling factor S suchthat a reference voltage V_(REF)=S*V_(BG) is developed. This referencevoltage is applied to one input of voltage regulator 48. The output ofthe voltage regulator 48 is coupled to the gate of transistor device 50.The source of the transistor device 50 is coupled to V_(DD) and itsdrain is coupled to a node of resistor 52. A node 58 between resistors52 and 54 (i.e. a series connection of two resistors) is coupled to asecond input to regulator 48 to provide a feedback loop to theregulator.

The N-bit input to DAC 40 is applied to logic 56 which outputs a numberof control busses including, by way of non-limiting example, controlbusses 60, 62 and 64. Control bus 60 can be used to control the value ofS of scale factor device 46, control bus 62 can be used to vary theresistance of resistor 52 and control bus 64 can be used to vary theresistance of resistor 54. The design and implementation of digitallycontrolled scale factor devices and variable resistors is well known tothose of skill in the art.

With continuing reference to FIG. 4, the voltage PA_(VDD) is given bythe following equation:

$\begin{matrix}{{PA}_{VDD} = {{V_{REF} \cdot \frac{R_{1} + R_{2}}{R_{1}}} = {V_{REF} \cdot ( {1 + \frac{R_{2}}{R_{1}}} )}}} & ( {{Equation}\mspace{14mu} 2} )\end{matrix}$

By way of non-limiting example, two bits or selection for voltage levelsV_(REF) and three bits of gain selection can be chosen for the feedbackto the regulator. Each increase in the V_(REF) voltage level willincrease the output power by 8 dB. Then the feedback resistors R1 and R2will gain the level by eight steps of 1 dB in this non-limiting example.

Table 1, below, illustrate the PA_(VDD) voltage levels and theircorresponding ideal and actual PA output power as determined by testingan example ADC 40 of FIG. 4.

TABLE 1 PAVDD Ideal PA Power Actual PA Power (V) (dBm) (dBm) 2.91 1514.5 2.60 14 13.8 2.314 13 13 2.054 12 12.3 1.833 11 11.3 1.638 10 10.31.456 9 9.3 1.30 8 8.3 1.12 7 7.0 1.00 6 6.3 0.89 5 5.0 0.79 4 4.0 0.703 2.8 0.64 2 2.1 0.56 1 0.8 0.50 0 0 0.45 −1 −1.0 0.40 −2 −1.8 0.356 −3−2.8 0.316 −4 −3.8 0.282 −5 −4.8 0.252 −6 −5.7 0.224 −7 −6.7 0.20 −8−7.7 0.179 −9 −8.7 0.16 −10 −9.7 0.142 −11 −10.5 0.126 −12 −11.7 0.113−13 −12.5 0.10 −14 −13.5 0.089 −15 −14.7 0.08 −16 −15.7

The DAC design of FIG. 4 is but one example of potential DAC designs. Aswill be appreciated by those of skill in the art, there a variety oflinear and non-linear DAC designs which can be used to achieve a desiredrelationship between the N-bit input and the power output of theintegrated circuit 38. It will also be appreciated that certaincomponents of system 36, such as the microcontroller 14 and/05 memory 16by way of non-limiting examples, can be integrated into integratedcircuit 38.

For example, instead of using an exponential voltage DAC, a linearvoltage DAC could be used with a look-up table to emulate an exponentialoutput or any other waveform. As another, non-limiting example, a buckconverter with feedback from an RF sensor on the output of the integratecircuit can be used. As yet another non-limiting example, a multiplyingDAC with a lookup table (stored in memory either on the integratedcircuit or off-chip) can be used. It will be appreciated by those ofskill in the art that the analog output of a DAC can be either a linearor a non-linear function of its digital input.

With reference to FIG. 3, if the impedance at the output 28 of theintegrated circuit 38 at a given operating frequency is RLpa, then theideal RF output power (PA) at output 28 can be expressed as:PA=PA _(VDD) ²/2/RLpa  (Equation 3)

When the RF output power is expressed in dBm, the relationship betweenthe RF power and the DAC setting isPA(dBm)=10*log(PA _(VDD)(V)²/2/RLpa(kΩ))=>PA(dBm)=10*log(k²/2/RLpa(kΩ))+20*log(e ^(qn))and, ultimatelyPA(dBm)=a+bn  (Equation 4)where

-   -   a=10*log(k²/2/RLpa(kΩ)) and    -   b=20*q/ln(e)

The relationship as set forth in Equation 4, above, indicates that thepower, when expressed in dBm, is linear with respect to the DAC inputword. This can been seen graphically in FIG. 5 with substantially linearcurves 60 and 68, taken at 25° C. and 125° C., respectively. Therefore,linear-in-dB power control is effectively achieved in this exampleembodiment.

In FIG. 6, the relationship of Equation 4 as set forth above indicatesthat the power when expressed in dBm is linear with respect to the valueof the DAC input word. The measured linear-in-dB characteristic wasdetermined to be accurate over almost three orders of magnitude ofoutput power. The output power characteristic was found to be accurateto within 0.2 dB over the full output power range and the variation overtemperature was found to be within +/−1.5 dB over the range of −40 C to125 C.

Another benefit of this power control method is that it offers improvedefficiency when compared to embodiments of the prior art which use fixedoutput power transmitters and attenuators to limit output power. In FIG.6, curve 74 is the drain efficiency of switching-mode power amplifier 34of FIG. 3 (“PA”) which is nearly constant over its power control range.The curve 72 illustrates a composite efficiency of the exponentialvoltage DAC 40 and PA. The prior art fixed output power PA followed byan ideal step attenuator with 0 dB minimum insertion loss, isillustrated by curve 74.

It should be noted that the embodiment of FIG. 3 is considerably moreenergy efficient than the prior art illustrated in FIG. 1. Maximizingefficiency is important in battery-powered applications such as inremote control and remote sensing.

While non-limiting examples refer to certain frequency ranges or bandsother embodiments may operate in other frequency ranges or bands. Someexample embodiments contemplate integration of circuitry on one or moreintegrated circuit chips, while other embodiments can be created fromdiscrete components or a combination of integrated circuit chips anddiscrete components. It will therefore be appreciated that althoughvarious embodiments have been described using specific terms anddevices, such description is for illustrative purposes only. The wordsused are words of description rather than of limitation. It is to beunderstood that changes and variations may be made by those of ordinaryskill in the art without departing from the spirit or the scope of thepresent invention, which is set forth in the following claims. Inaddition, it should be understood that aspects of various otherembodiments may be interchanged either in whole or in part. It istherefore intended that the claims be interpreted in accordance with thetrue spirit and scope of the invention without limitation or estoppel.

What is claimed is:
 1. A method for controlling a power output of an RFtransmitter comprising: developing a signal source; applying the signalsource to a buffer amplifier to provide an amplified signal; applyingthe amplified signal to a gate of a MOSFET having a source coupled toground and a drain providing a power output signal; and controlling again of the MOSFET with an analog signal in response to a digitalcommand including a plurality of bits, wherein the analog signal is afunction PA_(VDD)=ke^(qn) of the digital command where PA_(VDD) is aD.C. voltage on the drain of the MOSFET, k and q are constants and n isa number of significant bits of the digital command.
 2. A method forcontrolling a power output of a RF transmitter circuit as recited inclaim 1 wherein a relationship between the digital command and the poweroutput signal is linear-in-dB.
 3. A method for controlling a poweroutput of an RF transmitter comprising: applying a signal source to abuffer amplifier to provide an amplified signal; applying the amplifiedsignal to a gate of a MOSFET having a source coupled to ground and adrain providing a power output signal; and controlling a gain of theMOSFET with an analog signal in response to a digital command includinga plurality of bits, wherein the analog signal is an exponentialfunction PA_(VDD)=ke^(qn) of the digital command where PA_(VDD) is aD.C. voltage on the drain of the MOSFET, k and q are constants and n isa number of significant bits of the digital command.
 4. A method forcontrolling a power output of a RF transmitter circuit as recited inclaim 3 wherein a relationship between the digital command and the poweroutput signal is linear-in-dB.
 5. A method for controlling a poweroutput of an RF transmitter comprising: applying an amplified signal toa gate of a field effect transistor having a source coupled to groundand a drain providing a power output signal; and controlling a gain ofthe field effect transistor with an analog signal in response to adigital command including a plurality of bits, wherein the analog signalis an exponential function PA_(VDD)=ke^(qn) of the digital command wherePA_(VDD) is a D.C. voltage on the drain of the field effect transistor,k and q are constants and n is a number of significant bits of thedigital command.
 6. A method for controlling a power output of a RFtransmitter circuit as recited in claim 5 wherein a relationship betweenthe digital command and the power output signal is linear-in-dB.